Shot shell wads are used in shot shell cases to separate the powder or propellent charge from the shot charge and to provide a seal for the hot expanding gases produced by the burning propellent. Many early shot shell wads were made of a heavy, cardboard-like material and were installed in the shell directly over the powder charge. Such cardboard over-powder wads or card wads, stood up well to the heat of the burning propellent charge and provided a fair seal between the hot expanding gases of the burning propellent and the shot or pellet charge. Such card wads were often used in conjunction with filler wads placed between the card wad and the shot charge. The filler wads, made from a relatively soft material, such as fiber, pressed paper, or cork, absorbed the initial pressure surges caused by the rapidly expanding gases of the burning propellent. Consequently, the filler wads tended to "soften the blow" on the relatively soft lead pellets comprising the shot charge, thus reducing pellet deformation and improving their subsequent trajectory.
While such card over-powder wads and filler wads were used for a number of years, they were not without their disadvantages. For example, the inside wall of the shell case needed to be perfectly smooth and straight, otherwise leaks would develop between the burning propellent and shot, thereby reducing the muzzle velocity of the shot charge and causing other problems. Such wads also did nothing to protect the relatively soft lead pellets from contacting the inside wall of the barrel. Not only does direct contact with the barrel tend to deform the pellets, thus adversely affect the trajectory of the shot charge, it also increases barrel wear.
Another type of wad more recently developed is the plastic over-powder cup wad. While many different kinds of plastic over-powder cup wads exist and are being used, almost all include a powder cup, a collapsible mid-section, and a shot cup. The powder cup typically incorporates an obturating lip seal which provides for improved sealing under almost all conditions, especially with shell casings having tapered walls. The shot cup generally includes a plurality of thin plastic panels or petals that surround the pellets of the shot charge. The petals prevent the pellets from contacting the barrel of the gun during firing.
While such plastic over-powder cup wads are generally superior to the older style card and fiber wads and are used almost exclusively, they are still not without their drawbacks. For example, while the mid sections of many wads are intended to be collapsible, they often do not collapse by an amount sufficient to provide a significant cushion for the shot pellets. Consequently, such wads may cause significant pellet deformation with an attendant loss of ballistic consistency. Even if the collapsible mid-sections do provide sufficient cushioning, the deformation or collapse is not always uniform, which may subject the relatively soft pellets to rapidly changing acceleration loads, causing them to deform. Other types of collapsible mid-sections may collapse more on one side than the other, thus imposing side forces on the shot cup as the wad and shot charge travel down the barrel. Such side forces may cause the pellets to shift as they travel down the barrel, thus resulting in an asymmetrical shot charge and, of course, unpredictable pellet trajectories.
Another problem associated with plastic over-powder cup wads is that they can usually only be used with a specific shell and shot load. For example, a wad designed for use in a 12 gauge shell with a 1 oz. shot load usually cannot be used with a heavier shot load, such as a 11/8 oz. shot load. If such a heaver load is used, the collapsible mid section may be excessively deformed during loading and may not apply the proper pressure on the propellent charge. Consequently, a shot shell manufacturer or a shot shell re-loader must have on hand several different wads in order to produce shot shells with different shot loads.
Consequently, a need exists for a shot shell wad having an improved collapsible mid-section that can provide for more uniform deformation to avoid subjecting the shot charge to excessive or variable acceleration forces which may cause the pellets to deform and/or shift as they are accelerated down the barrel. Ideally, such a shot shell wad should be lightweight to reduce the mass that must be accelerated down the barrel. Additional advantages could be realized if such an improved shot shell wad could be used with different shot charges.